Exemplary embodiments of the present invention relate to a hybrid drive train, a hybrid vehicle having a hybrid drive train, and a method for operating a hybrid drive train.
A hybrid drive train includes at least one internal combustion engine and at least one electric motor. Both the engine and the motor are used for generating a drive power of the drive train, which may be utilized in a vehicle for generating propulsion.
German patent document DE 10 2007 016 551 A1 discloses a hybrid drive train having an internal combustion engine that may be operated in an auto-ignition operating process. The known hybrid drive train is operated in such a way that at low rotational speeds or loads the drive power is provided solely by the electric motor, whereas the internal combustion engine is switched off.
German patent document DE 11 2007 000 515 T5 discloses a method for controlling a hybrid drive train in which at least one set of route input data that include a starting point and an ending point of a preferred route of the hybrid vehicle is initially recorded. An optimal fuel-conserving route for reaching the ending point is subsequently determined on the basis of the at least one set of route input data. A drive train control strategy for achieving the optimal fuel-conserving route is then selected, the drive train control strategy including the determination of when the internal combustion engine and the electric motor are to be operated, and when a battery is to be charged. Lastly, the drive train control strategy is then carried out.
In internal combustion engines, a distinction is made among different combustion operating processes. Spark ignition operating processes operate using flame front combustion and are common in gasoline engines. Flame front combustion is initiated at an ignition point with the aid of an ignition device. The flame front spreads out spatially in the combustion chamber, starting from the ignition site. In contrast, auto-ignition operating processes basically function without an ignition device, and are generally characterized by charge compression combustion, in which the combustion reaction begins in the combustion chamber simultaneously at numerous spatially distributed locations. Auto-ignition operating processes are typical in diesel engines.
Whereas diesel engines are usually operated lean, in gasoline engines the mixture formation is generally stoichiometric. However, recent spark ignition operating processes for gasoline engines are known in which a lean mixture is likewise spark-ignited. For example, a lean mixture may be ignited via stratified charge in conjunction with direct injection.
Recent gasoline engines may likewise be intermittently operated in an auto-ignition operating process. A problem with gasoline engines is that these auto-ignition operating processes function reliably only at comparatively low rotational speeds and/or loads. At higher loads and/or rotational speeds, the auto-ignition operating processes are unstable and show a significant tendency toward knocking.
For stabilizing the auto-ignition operating processes, in gasoline engines it is possible in principle to initiate the charge compression combustion via spark ignition, as the result of which a desired ignition point may be achieved. By specifying the boundary conditions such as temperature and pressure, with the aid of spark ignition the framework necessary for reliable auto-ignition may be provided, so that the charge compression combustion may be directly triggered by means of the spark ignition.
Charge compression combustion has improved energy efficiency compared to flame front combustion.
Exemplary embodiments of the present invention are directed to an improved design, or at least a different design, for a hybrid drive train or a hybrid vehicle or associated operating method which is characterized by high energy efficiency.
The invention is based on the general concept of operating the internal combustion engine of the hybrid drive train in the range of lower and average rotational speeds and/or loads using a low-NOx operating method that is based on charge compression combustion, and in a range of higher loads, to operate in a spark ignition operating method that is based on flame front combustion. The low-NOx operating method is characterized in that a homogeneous lean mixture composed of air and fuel and optionally recirculated exhaust gas is spark-ignited at an ignition point by means of an ignition device in order to initiate a flame front combustion, the flame front combustion in turn initiating a charge compression combustion. Thus, in the low-NOx operating method the charge compression combustion is triggered twice: first by spark ignition by means of the ignition device, thus initiating the flame front combustion, and second by flame front combustion, which initiates the charge compression combustion. To be able to trigger the flame front combustion by means of spark ignition, the flame front combustion in turn triggering the charge compression combustion, the boundary conditions necessary for this purpose, such as temperature and pressure, must be provided in the particular combustion chamber, which in recent internal combustion engines is achievable using suitable measures.
The low-NOx operation of the internal combustion engine is characterized by comparatively high energy efficiency with low pollutant emissions, namely, low NOx emissions. However, the power output of the internal combustion engine in low-NOx operation is limited. For higher power, a change is made to spark-ignition operation, in which a homogeneous stoichiometric mixture composed of air and fuel and optionally recirculated exhaust gas is spark-ignited at the particular ignition point in order to trigger flame front combustion, which in contrast to the low-NOx operating method does not change over to charge compression combustion. For this purpose, the necessary boundary conditions such as pressure and/or temperature are provided in the particular combustion chamber, using suitable measures. The spark ignition operating method is characterized by a higher power output. However, less favorable efficiency and pollutant emissions must be accepted.
The operation of the hybrid drive train is advantageously carried out primarily in the low-NOx operating method, in which the electric motor in question may provide power assistance.
In one particularly advantageous embodiment a compression ratio is reduced for changing from the low-NOx operating method to the spark ignition operating method. In an internal combustion engine designed as a reciprocating engine, the compression ratio is understood to mean the ratio of the entire combustion chamber prior to the compression to the remaining space after the compression. Accordingly, the compression ratio is computed as the quotient of the compression volume (remaining space after the compression) and the sum of the displacement volume (difference between the volume prior to the compression and the volume after the compression) and the compression volume (volume after the compression).
For a high compression ratio, the energy-efficient low-NOx operating method may be implemented based on charge compression combustion. However, specifically because of the high compression ratio, the low-NOx operating method is limited to lower power levels, since instability such as knocking occurs at higher power levels. In contrast, reducing the compression ratio results in the range of customary spark-ignited flame front combustion, which is stable even at high loads but has reduced efficiency. An internal combustion engine in which the compression ratio is adjustable may be equipped with an appropriate device for changing the compression ratio. For example, the piston stroke may be variable.
In another advantageous embodiment, lifts of gas exchange valves may be increased with regard to amplitude and/or opening duration in order to change from the low-NOx operating method to the spark ignition operating method. To be able to provide boundary conditions in the particular combustion chamber that are suitable for the low-NOx operating method or for the spark ignition operating method, it may be necessary to control the gas exchange valves differently, in particular to change the lifts with regard to amplitude and/or opening time duration. For example, based on lifts of the gas exchange valves that are suitable for spark-ignition operation, it may be necessary to decrease the lifts with regard to amplitude and opening time duration in order to provide boundary conditions which are suitable for the low-NOx operation.
An internal combustion engine in which the lifts of the gas exchange valves are variable may be equipped, for example, with a device for adjusting the lifts of the gas exchange valves with regard to amplitude and/or opening time duration.
According to one particularly advantageous embodiment the internal combustion engine is operated in a quasi-steady-state mode in the range of low and average rotational speeds and/or power levels. The steady-state or quasi-steady-state operation of the internal combustion engine at varying low and average rotational speeds and/or loads is characterized in that, despite varying rotational speeds and/or loads, the particular operating point of the internal combustion engine is not varied or is varied only comparatively very little, so that the internal combustion engine is operated in quasi-steady-state mode in the operating point in question. In particular, in this quasi-steady-state operation of the internal combustion engine, only comparatively small sub-ranges for rotational speed and/or load are settable. The term “small” encompasses variations of 20% maximum, preferably maximal 10% maximum, of the maximum bandwidth of the particular range for the rotational speeds or for the loads. It is thus possible to always operate the internal combustion engine in a fuel-optimized operating state within the range having low and average rotational speeds and/or loads, as the result of which the internal combustion engine has particularly high energy efficiency.
If the required setpoint load exceeds the actual load achieved in steady-state operation of the internal combustion engine, the setpoint/actual difference may be provided by the at least one electric motor. The electric motor in question is supplied with power from a battery of the hybrid drive.
On the other hand, if the required setpoint load is below the actual load achieved by the internal combustion engine in steady-state operation, the setpoint/actual difference is decreased by the at least one electric motor, which in this case is operated as an electrical generator. It is thus possible, for example, to recharge the above-mentioned battery.
Furthermore, the situation may arise that the required setpoint load exceeds the actual load of the internal combustion engine in steady-state operation, and that in addition the setpoint/actual difference exceeds the maximum power of the at least one electric motor. In this case, a change is made from the steady-state operation of the low-NOx operating method to the spark ignition operating method; the internal combustion engine may then be dynamically operated in the spark ignition operating method. Due to the dynamic operation of the internal combustion engine in the spark ignition operating method, the internal combustion engine may be quickly adapted to changing rotational speeds and loads. The electric motor in question may also provide power assistance at these high loads and rotational speeds.
A hybrid drive train according to the invention includes the internal combustion engine and at least one electric motor, as well as a battery. In addition, the hybrid drive train includes a control system which is configured and/or programmed in such a way that it may carry out the above-described operating method.
A hybrid vehicle according to the invention, which may preferably be an on-road vehicle, includes a chassis, in particular a body or vehicle structure, in which a hybrid drive train of the above-described type is situated.
Further important features and advantages of the invention result from the drawings, and from the associated description of the figures with reference to the drawings.
It is understood that the features stated above and to be explained below may be used not only in the particular stated combination, but also in other combinations or alone without departing from the scope of the present invention.
Preferred exemplary embodiments of the invention are illustrated in the drawings and explained in greater detail in the following description; similar or functionally equivalent components are denoted by the same reference numerals.